Acoustic wave filters comprise a class of frequency selection components having the advantages of small size, light weight, and large out-of-band signal rejection.
Periodic or quasi-periodic acoustic wave transducer structures are employed for achieving acousto-electric and electro-acoustic energy conversion required in surface acoustic wave filters. The bandwidth of an acoustic wave transducer (and filter) is inversely proportional to the length of the acoustic wave transducer.
A first problem which all acoustic wave filters incur is that such structures typically provide efficient energy conversion, but also impose bandwidth limitations on the frequency response of the completed acoustic wave filter.
A second problem which many acoustic wave transducers suffer is "ripple" or nonuniformity of the pass-band frequency response of the completed filter due to what is termed "triple-transit" distortion of the filter response. Triple transit distortion results from acoustic reflections within an acoustic wave filter, which occur when a propagating acoustic wave impinges upon, for example, a simple, bi-directional acoustic wave transducer. Such transducers are often preferred because they are easily manufactured using a single photolithography step, by techniques which are very similar to those employed to fabricate semiconductor-based integrated circuit devices, as is well known in the art.
The triple transit distortion level can be minimized by increasing the pass-band insertion loss of the acoustic wave filter, without increasing the filter's fabrication complexity.
However, this approach requires large filter insertion losses in order to satisfy many system specifications for filter pass-band insertion loss uniformity. These losses are often deliberately introduced into acoustic wave filters in order to ameliorate triple-transit induced effects. In turn, these large filter insertion losses necessitate either pre- or post-filter gain, effected via amplifiers. Such amplifiers require power, occupy space, and impose weight requirements which are inconsistent with many applications for low-power, hand-portable communications equipment, wherein acoustic wave filters find substantial application.
What is needed are means and methods for achieving wide bandwidth acoustic wave filters. What is further needed are means and methods for providing wide bandwidth acoustic wave filters also having low pass-band insertion loss, low triple transit spurious response, strong rejection of out-of-band signals, and single level photolithographic process fabrication requirements.